Ultraviolet light—antimicrobial agents of the future

The part of sunlight in the ultraviolet spectrum plays an important role for our lives on Earth. With the new LED light sources and greater knowledge of the effect of ultraviolet rays, we are well on the way to being able to use UV light to improve human health. It is a well-established fact that ultraviolet light with a short-wavelength—the so-called UV-C rays—can kill bacteria. Now, researchers from DTU – Technical University of Denmark and the University of Copenhagen have shown that naturally occurring UV-B radiation has an even better antibiotic effect, which bodes well for the future assess DTU’s partners—the Department of Odontology—and Costerton Biofilm Centre at the University of Copenhagen.

Ultraviolet light—wavelengths from 400 to 100 nanometres—is located in the non-visible spectrum. UV-A-radiation has the longest wavelength, penetrating furthest into the skin, turning it brown—or possibly causing sunburn. With their shorter wavelength, UV-B radiation can cause greater damage. To a certain extent, UV-B radiation is mediated by the ozone layer. However, the part that reaches the Earth’s surface is key to producing the all-important vitamin D, which is linked to an increasing number of vital human functions.

UV-C rays have the shortest wavelength and are therefore the most dangerous. They can damage cellular DNA and are blocked by the ozone layer so they do not reach Earth. However, they can be recreated using LED lights and are currently used in the purification of water or the sterilization of surgical instruments.

Start with the worst

Professor Paul Michael Petersen from DTU Fotonik is focused on how the light can be used in a health context. Two years ago, he initiated a series of systematic experiments to identify precisely which wavelengths are the most effective in combatting bacteria.

For almost a century, we have used antimicrobial agents. However, increasingly bacteria are proving untreatable either because of resistance or because they join together to form so-called biofilms, organizing themselves so that they can withstand external influences.

Biofilm comprising many different bacteria can also form in an inflamed tooth root, which the dentist usually treats with mechanical cleaning and various oral irrigation fluids. This is an exhaustive process which does not always succeed—in some cases, the bacteria are not eliminated entirely—and in other cases, the infection resurfaces at a later date. In particular, the bacteria Enterococcus faecalis has proved difficult to treat—it is most often found in the root canal when the procedure has been unsuccessful. Dentists are therefore extremely interested in alternative, more effective treatments.

Another persistent bacteria often found in hospitals is Pseudomonas aeruginosa. Together with researchers from the University of Copenhagen, Paul Michael Petersen therefore decided to focus on the two aforementioned bacteria.

Researchers from DTU, Costerton Biofilm Centre, and the Department of Odontology—i.e. School of Dentistry—established a series of experiments in which biofilm containing either P. aeruginosa or E. faecalis at different stages of development were illuminated with diodes using different wavelengths.

Surprising result

“Basically, we wanted to test different UV-C diodes, as these rays are known to be the best disinfectants. But we also included some UV-B diodes, which we use to increase vitamin D levels in chickens and pigs. As it turned out, the UV-B radiation actually worked best. This came as a complete surprise to us and we had to repeat the experiments many times to be sure of our results,” says PhD student Aikaterini Argyraki, DTU Fotonik.

The researchers were in no doubt that they had to follow up on the surprising results. Even though UV-B light can damage the cells, it still is not as dangerous as UV-C, which would destroy the cells’ DNA if it come into contact with it.

The first attempts were made on biofilm, which the researchers created in the laboratory. They are now seeking permission to proceed with biofilm from patients requiring root canal treatment.

If it turns out that UV-B radiation works just as well on the biofilm from the patients’ root canals, the next step will be to find methods to guide UV light from the diodes into the root canal—perhaps using optical fibre technology.

“It will probably still be a supplementary treatment that can’t stand alone and it will be at least a couple of years before we reach this point. But UV-B light holds out great potential—not only in connection with tooth roots, but also in connection with chronic wounds, lung tissue infections, or around implants. All areas where you often encounter problems with resistant bacteria,” says Merete Markvart, who welcomes the fruitful interdisciplinary cooperation behind the exciting results.

► Teeth are one of the areas in the body where biofilm that can be combated with UV-B light typically forms. Other sites may be urinary catheters or heart and hip implants.

“We envisage inserting a cone into the root, which in the course of three weeks becomes embedded in the bacterial biofilm. The patient won’t feel anything, but when the cone is removed we will have a natural biofilm of different bacteria to work with—in contrast to the monoculture we have been able to create in the lab,” says Merete Markvart, assistant professor at the School of Dentistry.

In the laboratory, the researchers have succeeded in growing the bacterium P. aeruginosa as a biofilm on filter paper. The bacterium strain has been designed in such a way that it expresses the fluorescent protein GFP (green) and is stained with propidium iodide (red)—which colours only dead cells. The biofilms are then treated with either UV-B or UV-C light. In the far right-hand side of the picture you can see a clear reddish discoloration of the biofilm.This means that UV-B treatment has killed a larger part of the bacteria in the biofilm than the UV-C treatment—in the centre to the right—where you can only discern the red colour.